"Atomic Bomb" is the correct answer. The United States became the first country to employ an atomic bomb in warfare, to great--if very controversial--success. The bombs dropped by the Enola Gay (the plane carrying both bombs) cause unprecedented destruction and eventually led to the surrender of the Japanese Empire, and the complete end of WWII.

The Cold War, which waged from the end of the Second World War until the fall of the Soviet Union, was primarily a conflict between capitalism and communism. It was essentially a conflict to see which economic system would become dominant in the world following the Second World War. The terms "democracy" and "socialism" are the two political systems that usually match with the economic systems of capitalism and communism, although it is possible to have a society that is both democratic and communist.

The first leader of Communist China was Mao Zedong. Mao was the founder of the Chinese Communist Party and the first ruler of the People’s Republic of China, which he governed from 1949 until 1976—when he died. Mao is notable for his role in the Chinese Civil War, leading the attack against the nationalists led by Chiang Kai-Shek. He is also notable for committing numerous atrocities, such as the Cultural Revolution, and for instituting the Great Leap Forward, designed to modernize the Chinese economy and solidify the hold of communism.

Mikhail Gorbachev, the last leader of the Soviet Union, ushered the country out of the Cold War and into an era of reforms and diminishing influence. His two major reforms are referred to as Glasnost and Perestroika. Glasnost was Gorbachev’s attempts to allow for greater political freedom; Perestroika was his attempt to restructure the soviet economy.

Joseph McCarthy was a United States senator in the late 1940s and early 1950s who famously led the crusade against prominent American communists. This period of American history is often referred to as The Red Scare, and the policy of attacking any known or suspected communists in Congress is called McCarthyism.

The Nationalist Party was led by Chiang Kai-Shek. During the Chinese Civil War the Nationalists faced off against the Communists, led by Mao Zedong. The Nationalists were eventually defeated and set up government in Taiwan.

NATO (The North Atlantic Treaty Organization) emerged after the end of the Second World War as the primarily alliance of the capitalist world during the Cold War. It was opposed by the Warsaw Pact, which was signed by the various communist nations of Europe and the Soviet Union. The current state of Russia remains excluded from NATO.

Mutually Assured Destruction is the term used to describe the nuclear relationship between the United States of America and the Soviet Union during the Cold War. Essentially it suggests that because both nations possess stocks of nuclear weapons, warfare is considerably less likely, because if one nation launches a nuclear attack on another nation it will result in the destruction of both nations. Nuclear Non-Proliferation refers to an agreement that no new nations will be allowed to access nuclear weaponry.

Disarmament is the term used to describe an agreement between two or more nations to downgrade their stockpile of weapons. Particularly, it is used to describe agreements between the United States and the Soviet Union during the Cold War.

In exchange for the Soviet Union removing its missiles from Cuba, during the Cuban Missile Crisis, the United States removed its missiles from Turkey. Its possible you did not know this information, but you should have been able to make a reasonable guess based on the answer choices available. Turkey was a major ally of the United States and Western Europe during the Cold War and was a member of NATO, whereas East Germany, China, Poland, and Czechoslovakia were all countries under either the direct control of the Soviet Union or, in the case of China, under a communist government.